326 



CONDUCTING SYSTEM 



3 



J) 



limiting membranes of which undergo active surface growth and thus 

 push their way into the cavities of the vessels ; in annular and spiral 

 vessels, tyloses similarly arise from circumscribed areas of the thin 

 membrane separating two adjacent thickening rings or two successive 

 turns of a spiral thickening fibre (Fig. 132). A single parenchymatous 

 cell may give rise to several tyloses. As long as it remains alive, 



of course, a tylosis contains cytoplasm 

 and cell-sap, while it may also be 

 furnished with a nucleus which has 

 passed over from the parent cell. The 

 walls of tyloses are generally thin ; 

 where adjoining tyloses become con- 

 crescent, the common walls generally 

 bear pits which correspond on both 

 sides in the ordinary way. Hermine 

 von Eeichenbach did not fail to note 

 the fact that tyloses are but rarely 

 cut off from their parent cells by septa. 

 Molisch was also unable to discover any 

 septation, except in Guspidaria ijtero- 

 carpa and in Rdbinia. Multicellular 

 tyloses, closely resembling hairs, are 

 recorded by Winkler as occurring in 

 Jacqucmontia violacea. As a rule, 

 however, these structures do not consist 

 of independent cells, but represent mere 

 diverticula of parenchymatous elements. 

 The functions of tyloses seem to 

 be somewhat variable. They occur 

 with especial frequency beneath the 

 scars of branches that have broken 

 off, and also near the severed ends 

 of cuttings, being in both cases 

 developed in such numbers that they become flattened by mutual 

 pressure, and completely fill the cavities of the vessels for a consider- 

 able distance ; in these instances it can hardly be doubted that they 

 serve, as Boehm first suggested, to plug up the cavities of the conduct- 

 ing tubes that have suffered mechanical injury. The tyloses which 

 appear in heart-wood, or in the older portions of splint-wood, in the 

 absence of any injury, apparently fulfil a similar purpose. Sclerotic 

 tyloses, such as occur in Piratinera guianensis and Mespilodaphne 

 Sassafras, must obviously add to the mechanical strength of the heart- 

 wood. Those tyloses, finally, which contain starch grains, like the cells 



Fig. 132. 



Tyloses. A. Young tylosis developing in 

 a spiral vessel of the petiole of Monstera 

 ddiciosa. B. Tylosis in a pitted vessel of a 

 branch of Robinia Pseudacacia. D. A tylosis 

 just beginning to develop in an annular 

 vessel of the petiole of Cucurbita maxima. 

 C. A similar tylosis at a later stage of 

 development. 



